Pneumatic casing tools are gripping devices used to hold and lower tubes or tubular well casing into a pre-drilled hole. The tools are used in sets consisting of one elevator slip assembly and one spider slip assembly. The elevator and spider slip assemblies are functionally identical tools except for the accessories used to operate each tool. A problem associated with the use of these tools is related to gripping the casing collar which is of a larger diameter than the outside diameter of the well casing. The problem is caused when the elevator slip assembly is not lowered sufficiently below the collar. The slip assemblies are designed such that the gripping forces generated are sufficient for proper gripping only when the slips are lowered sufficiently below a casing collar so as to completely grip the outside diameter of the well casing and not the collar. When the collar is gripped, the slips will not sufficiently engage with the casing to generate adequate gripping forces. The result is that partial engagement of the slips against the casing string may result in the casing slipping from the tool and dropping into the well bore causing significant down time and repair.
The person working up in the derrick, called the "stabber", operates the control valves that close the elevator slips. Once the elevator slips are closed and the weight of the casing is on the elevator, the stabber sometimes actuates the control valve to the open direction. However, with the casing weight hanging on the elevator, the air pressure alone will not open the slips. The proper time to actuate the control valve is after the string is lowered and the spider assembly slips are closed, and not before.
There is an instance when this is a problem. This instance would occur when the casing is being lowered into the well bore and meets up with some restriction or abutment which prevents downward movement of the casing. The elevator, however, continues to move downward a short distance because of the reaction time of the driller who is controlling movement of the tool. This situation is a problem when the slips have been actuated in the open direction but have been held down by the weight of the casing. The weight is no longer on the elevator and the slips consequently open up. If the casing should suddenly free itself in this manner and drop, neither the spider nor the elevator are in the closed position and the casing drops into the well bore.
Another problem is that once an elevator or spider has been energized to the open or closed position, there is a time required to allow the tool to reach the gripped position, detect that this has occurred and have the interlock system respond accordingly. During this time the interlock system may not function properly.
Flush mounted spiders utilize a series of hydraulic cylinders rather than pneumatic cylinders to power slips upward to the open position or downward to the closed position. Of particular danger, which is unique to the flush mounted spider, is the ability of the spider slips to be opened inadvertently despite being engaged in the down position with casing suspended in the slips. This is possible due the substantial upward force which can be applied to the slips thus dislodging them from the closed position. The substantial force is the result of the high operating pressures that are typical of hydraulic systems (2000 to 3000 psi) as opposed to the lower operating pressures (80 to 150 psi) that are typical of pneumatically operated elevators and spiders. Additional problems arise due to the fact that the operational controls for this spider are located within a separate control panel as opposed to being mounted on the tool itself.
Pneumatic conduits between the elevator and spider are typically about 120 feet long and 3/4 inch in diameter. The fluid volumes from such conduits are large and the response to operation of control valves may be sluggish, possibly endangering the operator. The present invention includes pressure circuits where conduits that would have been 3/4 inches in diameter may be about 1/2 inch in diameter instead, and conduits that would have been 120 feet long are now about three feet long. The smaller conduit lengths and diameters allowed by the present invention reduce the fluid volumes that must be handled by the apparatus. Smaller fluid volumes, in turn, result in improved response time and safer operation of the apparatus.
The pertinent and presently known prior are to this invention are U.S. Pat. Nos. 3,215,203; 3,708,020; 3,722,603; 4,676,312; 4,842,058; and 5,343,962, as well as Varco BJ Oil Tools Brochure entitled FMS 375 Flush Mounted Spider.